Domestic ducks could be playing an important role in spreading bird flu in Asia, warns the World Health Organisation (WHO).

Researchers have shown that ducks infected with forms of the virus circulating in 2004 retain the bird flu virus for longer and excrete more of it, than those infected with viruses from 2003. Most of the infected ducks showed no signs of illness, making the infections difficult to detect. The study also shows that recent strains of the virus persist in the environment for several days longer than earlier strains.

So far, most human cases of bird flu have been linked to birds, including domestic ducks, that show signs of disease. However, there have been some human cases where the origin of their infections could not be traced. Based on the new information, the WHO has issued updated public health recommendations.